The shlöka II.3.6 of Aitareya Ārāṇyaka explains this :-
……. ‘a’ is the whole of speech and being manifested through the mutes and the sibilants it becomes manifold and various. If uttered in a whisper, it is this ‘prāṇa’, if forcefully, that body – ‘sharira’. Therefore it is hidden, as hidden as the previous body encapsulated in this prāṇa . But spoken forcefully it is that body and visible, for body is visible.Now ‘a’ ( A ) is the immutable, the symbolic Unknown – if this is the whisper, then as we force the breath just a little, what we get is the aspirate sound ‘ah’ ( A: ) . It is both ‘a’ and ‘h’ combined. This is called the visarga sound. As shown in the akshara-wheel , at the top, the visarga is not included in the main alphabet and hence it is termed ayogavāḥ or ‘that which is not part of the harness’. The visarga is the bija-mantra for the Sahasarāra or the Crown-chakra. Here we see that another level of the symmetry emerges – ‘ah’ ( A: ) closes the circle pictorially – signifying the internal/external division of the body or ātma. The individual ‘I’ has thus taken form and this is called ‘aham’ ( AhM ). It is the ‘ah’ ( A: ) sound with the Shiva bindu on top of it. Thus there is an inside and there is an outside – with the language of consciousness as the dividing membrane!